Patent GB-703103-A (GB)
Title: Television system employing horizontal dot interlacing
IPC: H04N11/12
Filed: 1951-07-16
Abstract: 703,103. Television. PHILCO CORPORATION. July 16, 1951 [July 22, 1950], No. 16837/51. Class 40 (3) In a television system in which the reproduced image is composed of dots which are interlaced in both horizontal and vertical directions, a horizontal interlacing operation is brought about by periodically varying the phase of the line synchronizing pulses while the phase of the dottiming signal remains constant. In Fig. 1, which illustrates the invention as applied to a threecolour system, camera 10 operates on a 60-field, 30-frame basis to generate simultaneous red, green and blue colour signal trains which are applied through filters 16, 20, 24 to modulators 18, 22, 26 where they are sampled in known manner at equally-spaced time intervals by 120 degrees-phased-displaced waves at a frequency of 3.19 Mc. The combined outputs from the modulators pass through filter 34, equalizer 36 and are then applied to modulate the transmitter 38. Bursts of signal at 3.19 Mc are added to the combined wave during line blanking intervals by circuits 42 and 44 to synchronize the corresponding sampling device employed at the receiver. A synchronizing generator 14 applies blanking pulses to the camera, times the 3.19 Mc burst injection circuits 42, 44 and adds line and frame synchronizing signals to the composite wave train. In order to lock the operation of the synchronizing generator with the dot-timing signal, a common crystal oscillator 30 controls both operations, the 3.19 Mc dot timing signal being derived by frequency divider 28 and the synchronizing generator being locked by a 94.5 Kc signal from frequency divider 48. In accordance with the invention, the time position of the synchronizing pulses is shifted relative to the timing wave by one-quarter the period of the dot frequency (“ Î 3.19 Á sec) during alternate field periods. To effect this, the output from divider 48 feeds the synchronizing generator through two gate circuits 50, 58, the connection to gate 58 including a delay circuit 56, and the gates being opened alternately by a 30 cycle square-wave keyer 52 triggered by the vertical timing pulses from the synchronizing geenrator. The presentation on the screen of the receiver cathode-ray tube takes the form shown in Fig. 2a. The screen is divided vertically into green, red and blue phosphors as indicated by the letters G, R and B. The dots laid down in four successive fields are indicated by the numbers 1-4. The horizontal off-setting of dots in successive line-scannings of the same field is due to the choise of dot-timing frequency which provides n + ¢ dots per line, and cancels out at the end of each four successive fields. The dots in fields 2 and 4 are shifted by one quarter of the dot period due to the shift in the timing of the synchronizing pulses. For monochrome television, the system shown in Fig. 1 is modified by eliminating two of the three componentcolour channels and increasing the frequency of the dot-timing signal applied to the remaining channel to 9.57 Mc. (3 X 3.19 Mc). The delay of circuit 56 is reduced to one third (38.28 Á sec) to allow for the fact that successive dots in each line scan now represent the same " colour " characteristic of the image. Another dot structure for monochrome or colour may be obtained by shifting the horizontal synchronizing pulse timing by one-third of the dot period in alternate frames. In a colour system this does not reproduce the colours in vertical columns as shown in Fig. 2a and the cathode-ray tube must be provided with coloured phosphor dots instead of stripes.